The invention relates to guidance devices for self-powered vehicles and more particularly to a magnetic wire-following guidance device for an order picking vehicle or the like.
In the material handling industry, high-rise order picker vehicles (OPVs) permit narrow aisle storage and retrieval operations of non-palletized case or item storage. Such OPVs carry an operator on a lifting platform who picks orders from either a pellet or a storage module. The lifting platform incorporates the vehicle control so the operator can ride on the platform. The aisle widths are extremely narrow and may be as narrow as four feet. If the vehicle is guided other than by the operator down the aisle, it is safe for him to drive forward while the platform on which he is standing is being elevated. Otherwise, in non-guided aisles, the driver must first drive to the proper location and then activate the lift. This feature of moving forward while elevating, called "overlap" in the material handling industry, significantly improves operator productivity. Industrial safety codes in many states do not permit overlap unless the vehicles are guided in the aisle other than by the operator. OPV guidance, then, not only achieves increased storage capacity by minimizing aisle widths, but is also increases labor productivity by speeding up picking times.
There are basically two types of prior art guidance systems for OPVs. In the first type the OPV is mechanically guided. Rollers attached to the vehicle make contact upon aisle entry with steel rails bolted to the floor on either side of the aisle. The operator controls speed, stops and starts, and the vehicle is centered in the aisle mechanically. Although mechanical guidance systems offer economy and some degree of flexibility over the electronic guidance systems to be discussed hereinafter, the mechanical guidance systems have several disadvantges, such as high maintenance cost, rough operation, and excessive space requirements.
Many of the above disadvantages of mechanically-guided OPVs are overcome by fully electronic guidance systems in which a wire is buried in the floor of the storage facility and specialized sensing positioning units are built into the OPV to follow along the buried wire. Such specially-built OPVs may run as much as $50,000 to $75,000, which is many times more expensive than mechanically-guided OPVs. Most of such electronic systems are completely automated, that is the vehicle is under automated control all of the time. Because of this enormous expense, mechanical quidance, at the present time, has nearly all of the OPV guidance market. One advantage of electronic guidance is that it minimizes vehicle down time and maintenance expense due to mechanical damage to equipment. This is because in the electronic guidance system, the OPV is actually steered down the aisle rather than being mechanically forced into position.
Ideally a low cost guidance system would combine manually steering and automatic steering at the operator's election. A problem arises in making the transition from manual to automatic modes in that the vehicle must be properly positioned over the buried wire for the sensing coils to be fully energized by the wire. This procedure of "acquiring" the wire, at least in fully automatic systems, has heretofore required that the vehicle approach the wire within a wide access course and generally that it actually cross the wire before the automatic guidance circuit could be activated. This requires unnecessary space in the storage facility and also consumes unnecessary amounts of the operator's time.